The harvesting of solar energy is a field with a multiplicity of different technologies for converting sunlight to electricity. To date, none of the technologies has become sufficiently inexpensive to displace traditional means of generating electricity, and as a result solar energy remains a marginal contributor to global power needs. The main cost driver in solar power systems is the high cost of the photovoltaic (PV) cells, which are the semiconductor junctions that convert light into electricity.
One of the many avenues being investigated for reducing the cost of electricity produced by solar power is called “Concentrated Photovoltaics”, or CPV. The basic idea behind CPV is to use some sort of optic, generally a Fresnel lens or another focusing optic, to concentrate sunlight onto tiny, high-efficiency PV cells. The PV cells employed are compound semi-conductor cells with multiple junctions in a stack and electrically connected in series. The most typical convetional cells for CPV are three junction cells using indium gallium phosphide, indium gallium arsenide, and germanium cells all arranged in series. Each of these cells converts a portion of the solar spectra into electricity. These systems are very energetically productive but they have a major downside in that they require trackers to orient them to face the sun at all times in order for their optics to function. This need for trackers makes these systems practical for use in solar farms, where large post-mounted trackers are mounted on the ground. Trackers are impractical, however, for systems intended for building integration and roof mounting (which represents a massive portion of the solar market). CPV systems use high sunlight concentration, as high as 2000 suns, meaning that only a tiny amount of photovoltaic material would be required as compared with a conventional non-concentrated PV system.
Another approach to concentration is the use of luminescent solar concentrators. These devices consist of a sheet of glass that contains either a layer of luminescent particles or has luminescent particles impregnated throughout the glass. Luminescent particles absorb light over a wide band of frequencies and emit light at lower frequencies over a narrower band. Examples of luminescent particles are organic dyes, laser dyes and nano-crystals.
When these luminescent particles emit light, the light emitted travels in a random direction. Because this light is emitted evenly in every direction from inside the glass, any emitted radiation which strikes the top of bottom faces of the glass sheet, and which has an angle of incidence with respect to the surface normal of the glass sheet greater than the critical angle for total internal reflection, will be trapped within the glass sheet by total internal reflection. (If the glass has an index of 1.5 and the surrounding media is air then the critical angle is approximately 41.8 degrees.) In fact, the only light which will not become trapped within the glass is any light that is emitted within one of two cones of emission centered on the normal of the top and bottom glass surfaces and with base angles of 83.6 degrees.
Light thus trapped will travel in all directions within the glass to the four edges of the glass where it can be harvested for energy production by photovoltaic cells. Because the frequency of the emitted light is relatively narrow, it is possible to use single junction cells in this instance in a very efficient manner, provided the single junction cells have the correct band gap. In principal, infinite concentration could be achieved in this manner except there are two fundamental limitations: absorption within the glass and re-absorption by the luminescent particles. The first, absorption within the glass itself, limits the practical optical path length and thus the size of the glass sheet and the concentration. Re-absorption and emission also limit the practical concentration. To date the best-predicted concentration by this means is on the order of 150 suns. This is far lower that the concentrations achievable by CPV as noted above. Thus cost savings in a luminescent concentration system achieved by not having a tracker are greatly overwhelmed by the extra cost of requiring several times more photovoltaic cell material. Thus, to date, luminescent concentration systems in are not in widespread commercial use and improvements in this technology are desirable, given its inherent advantages noted above.